PLEISTOCENE EXPOSURES IN DES MOINES 63 
tocene topography we can form some definite ideas. Following 
the retreat of the Nebraskan ice-sheet and the development of the 
Nebraskan gumbotil a system of streams must have been deeply 
carved into the unconsolidated glacial deposits and in many cases 
into the underlying Paleozoic strata. Such Avas the case at Des 
Moines and it resulted in the formation of a deep broad valley 
from whose slopes apparently much of the till was removed. The 
Raccoon river must have been a stream of large importance in 
those days as now^ for the wide valley south of the State House 
originally belonged to that stream. Again after the recession of the 
Kansan glacier a similar sequence of events occurred. Before the 
loess was deposited the old valleys had been re-excavated, or new 
ones formed, and the Kansan till had been swept away from some 
parts of the surface, permitting the formation of several feet of 
geest from the shales of the Coal Measures. The loess still formed 
a blanket to the valley's edge when the Wisconsin ice covered the 
region, and protected it from further weathering or erosion. To 
the south of the river the loess still forms the surface covering. It 
is of interest to note here that in the early part of 1921 an elephan- 
tine tusk measuring nearly ten feet in length was found buried 
in the loess in the south part of Des Moines. It is certain then, 
that its owner was roaming over the Iowa prairies as recently as 
Peorian time, during the period in which human beings have been 
living, in the Old World at least, if not in the New. 
Iowa Geological Survey 
